662 



MAMMALIA. 



In sludying the skull it is convenient to consider the bones in groups. 



On the posterior surface of the skull, the foramen magnum, through 

 which the spinal cord issues from the cranial cavity, is bounded by the 

 basi-occipital beneath, the ex-occipital on the sides, the supra-occipital 

 above. The ex-occipitals form most of the occipital condyles, but the 

 basi-occipital contributes a small part. In many mammals the ex- 

 occipitals alone form the condyles. From each ex-occipital a par-occi- 

 pital process descends and is applied to the tympanic bulla — a dilatation 

 at the base of the tympanic bone which protects the external auditory tube. 



Along the roof of the skull from behind forwards lie the supra-occipi- 

 tal, the parietah, the frontah, and the nasals. Between the supra- 

 occipital and the parieials there is a small i7iterparietal. 



On the very front of the skull are the p7-e7jiaxilla: bearing the incisor 

 teeth. Behind each preinaxilla is a maxilla, bearing the premolars and 

 molars, behind this, along the zygomatic or temporal arch projecting 



Pmx. 



Fig. 230. — Side view of Rabbit's skull. 



Pmx., Premaxilla; Na., nasal; ^r., frontal ; Pa., parietal; Sq., 

 squamosal; S.O., supra-occipital; Pci:, periotic ; T., tympanic; 

 P.O., par-occipital process. 



beneath the orbit is theyV/^(z/or i/ialaj- which unites posteriorly with the 

 squamosal. This zygomatic arch bridges over the deep temporal fossa 

 behind the orbit, and serves for the insertion of muscles, and its 

 " squamoso-maxillary " structure occurs outside of ]\Iammalia in the 

 Anomodont reptiles only. The squamosals form a great part of the 

 posterior side walls of the skull, and articulate with the parieials, 

 f?'0}itals, orbitosphenoids, and alisphenoids. At the posterior end of 

 the zygomatic arch is the longitudinall)' elongated glenoid cavity in 

 which the mandible moves backwards and forw ards. 



In connection with the floor of the skull and the roof of the mouth, 

 there lie from behind forwards the following components : — the median 



